I. Field of the Invention
The present invention relates generally to bootstrap hydraulic systems and, more particularly, to enhancing such systems for use in vehicular power steering systems.
II. Description of the Prior Art
In bootstrap hydraulic systems of the prior art as applied to vehicular power steering systems, fluid is supplied by a pump to a nominally balanced, closed-center, four-way control valve (hereinafter being referred to as a closed-center control valve) comprising input, return, and first and second output ports. The input and return ports are coupled to input and return slots, and the first and second output ports are coupled to first and second output slots formed in valve input shaft and sleeve members, respectively, comprised in the closed-center control valve. Control valve input and return flow control orifices are formed by progressive overlapping of input and first or second output slots, and second or first output slots and return slots, respectively, as a result of differential positioning of the valve spool and sleeve members (hereinafter being referred to as valve positioning or valve position). A first direction of valve positioning generates first input flow control orifices allowing fluid flow between input and first output ports, and second return flow control orifices allowing fluid flow between second output and return ports. The second, or opposite, direction of valve positioning generates second input flow control orifices allowing fluid flow between input and second output ports, and first return flow control orifices allowing fluid flow between first output and return ports. In either case, each open flow control orifice becomes an individual flow source having fluid flow therethrough equal in value to EQU Q.sub.o =A.sub.o C.sub.d (2.increment.P/.rho.).sup.0.5 ( 1)
where Q.sub.o is fluid flow through a particular flow control orifice, A.sub.o is effective flow area of the flow control orifice, C.sub.d is discharge coefficient of the flow control orifice, .rho. is density of the fluid used and .DELTA.P is nominal pressure drop across any of the open flow control orifices.
Most bootstrap hydraulic systems utilize square cut input and return slots which generate rectangularly shaped flow control orifices as a function of valve positioning. The flow control orifices are generated as a function of input and return slots overlapping respective ones of the first and second output slots. This results in volumetric fluid flow through the open ones of the flow control orifices being a linear function of the valve position. Thus, open ones of the closed-center control valve's input and return flow control orifices act as flow sources proportional to valve position and the combination of open ones of the flow control orifices may be thought of as a proportional flow source providing fluid flow directed toward one of the output ports and away from the other.
In order to attain a desired value for .DELTA.P, fluid is supplied in a controlled manner whereby pump delivery pressure is maintained equal to differential load pressure values imposed between the first and second output ports (i.e., by a steering load) plus a substantially constant supplemental pressure equal in value to twice the value of .DELTA.P. This results in a required value for pump delivery pressure according to the equation P.sub.S =P.sub.L +2 .DELTA.P where P.sub.S is pump delivery pressure and P.sub.L is differential load pressure.
In bootstrap hydraulic systems of the prior art powered by an engine driven positive displacement pump assembly, this is accomplished by using a three-way valve to pick off the higher valued one of the first and second output port pressures, which pressure is conveyed to the pump assembly via a control line and applied as a control pressure to the control end of a piston normally utilized as a flow control valve for the pump assembly. The control pressure value is supplemented by force concomitantly provided by a compression spring also acting upon the control end of the piston in order to generate pump output pressure equal in value to the higher valued one of the first and second output port pressures plus .DELTA.P. Because of the nominally balanced nature of the closed-center control valve, the lower valued one of the first and second output port pressures is substantially equal to .DELTA.P. This results in a pump delivery pressure equal to the above mentioned value P.sub.S.
In bootstrap hydraulic systems powered by a servo motor driven positive displacement pump, an equivalent pump delivery pressure is generated by using a three-way valve to pick off the lower valued one of the first and second output port pressures, which pressure is applied as a control pressure to a pressure transducer. An electronic signal emanating from the pressure transducer is utilized as an input to a controller which, in turn, drives the servo motor such that the lower valued one of the first and second output port pressures is maintained equal in value to .DELTA.P regardless of load pressure values. Because of the nominally balanced nature of the closed-center control valve, the difference between the pump delivery pressure and the higher valued one of the first and second output port pressures is also substantially equal to .DELTA.P. This results in a pump delivery pressure equal to the above mentioned value P.sub.S.
In order to avoid power cylinder hydrostatic lock when valve position has zero value, as well as generating useful pressure-effort characteristics when it has non-zero value, parasitic slots are usually formed in valve spool members in bootstrap hydraulic systems of the prior art when they are used for bootstrap power steering systems.
The parasitic slots are formed in an open-center manner comprising
first and second parasitic orifices formed by overlapping of parasitic and first and second output slots, respectively. As a result, fluid flow is permitted through the output ports via series arrangement of the first and second parasitic orifices when valve position has zero value. Further, the first and second parasitic orifices can pass selected portions of the above mentioned linearly generated fluid flow directly through the closed-center control valve when valve position has non-zero value as long as both remain open. In general, this results in a particular pressure-effort characteristic curve for any selected load flow value (i.e., fluid flow delivered from the first output port and returned to the second output port) attainable with valve positions whereat both first and second parasitic orifices are open. For valve positions beyond where both first and second parasitic orifices are open, pressure-effort curves become straight lines denoting values of effort required for compatible values of flow control orifice areas. The resulting family of such pressure-effort curves is characterized by unusually wide effort separation values between individual curves.
Bootstrap power steering systems comprising bootstrap hydraulic systems have exceptionally smooth and precise operational characteristics when compared to open-center rotary valve equipped power steering systems. Intuitively, this should be expected because the above noted proportional flow source nature of open ones of the flow control orifices emulates the functioning of flow control orifices in a closed-center industrial control valve, and industrial hydraulically powered servo systems comprising such control valves are known to be exceptionally smooth and precise. The operational smoothness is also depicted by the above noted family of pressure-effort curves. The wide separation of the curves denotes exceptionally high damping values, whereby damping is indeed perceived by a driver of a host vehicle. This is because succeeding curves represent increasing steering wheel rotation rates whereby their widely separated effort values denote high supplementally applied torque to rotational velocity ratios. The rate of change of steering wheel torque with respect to increasing values of steering wheel rotational velocity can be defined as perceived steering wheel damping coefficient.
In bootstrap hydraulic systems of the prior art, it has been necessary to utilize a pressure dividing network comprising substantially equal valued pilot flow orifices disposed in series whereby input and control pressure ports, and control pressure and return ports are respectively coupled. This effects substantially balanced operation of the pressure control function when the valve position has zero value because the pilot flow dominates commonly encountered leakage flows and obviates an otherwise critical need to balance the leakage flows. However, the pilot flow is undesirable because it is subject in part to substantially full system pressure and places a parasitic load on the bootstrap power steering system which can be significant at higher pressures commonly encountered in parking maneuvers. It would be desirable to delete the pilot flow function when the bootstrap hydraulic system is operated at the higher pressures or even eliminate it entirely.
Other desirable bootstrap hydraulic system enhancements include advanced input, return and parasitic slot geometries which would allow a more comprehensive selection of pressure-effort characteristics. Further, it would be desirable to provide a variable assist feature for bootstrap power steering systems utilizing bootstrap hydraulic systems to enable speed sensitive steering as an available option. Finally, it would also be desirable to minimize energy consumption. The pump bypass function discussed above can be eliminated by providing improved variable displacement pump apparatus. Efficiency can also be increased by eliminating half of the supplemental pressure value of twice the value of .DELTA.P such that a required pump delivery pressure would be reduced to a value determined by the equation P.sub.S =P.sub.L +.DELTA.P.